1. Field of the Invention
The present invention relates to a method of manufacturing a device, a device manufactured thereby, a computer program for controlling a lithographic apparatus, and a lithographic apparatus.
2. Brief Description of Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, such as a mask, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning” direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
When exposing a plurality of rectangular dies on a circular (except for the flat, if provided) substrate, it will be appreciated that not all of the substrate can be filled with whole dies; around the edges so-called mouse bites would be left unexposed. To ensure uniformity in subsequent processing, it is customary to perform exposures of dummy structures in the areas where complete dies cannot be fitted. The purpose is that complete dies near the edge of the substrate have neighborhoods that appear similar to the neighborhoods of dies in the center of the substrate and thus their subsequent processing is more similar. Otherwise, no structures would build up on the unexposed areas and the unexposed areas would remain smooth and after several process layers would be at a different level than the exposed areas. This can lead to undesirable effects such as variations in resist thickness at the edges, since the resist runs off more easily over the smooth unexposed areas, incorrect level measurements and/or undesirably abrupt leveling movements during a scan. It can also be desirable to expose dies that overlap the edge of the substrate if the mask contains more than one device and a whole device or devices, but not the whole mask image, can be accommodated by an edge die. The additional devices that are made in this way increase yield.
However, if exposures that overlap the edge of the substrate are carried out, the beam of radiation will fall on the substrate table leading to undesirable heating thereof. The substrate table will in general carry mirrors for the interferometric displacement measuring system (for which reason it is often referred to as the mirror block) which measures movements of the substrate table and these mirrors are maintained flat to a very high degree of accuracy. Localized heating of the substrate table leading to thermal expansion can distort the mirrors and cause errors in the measurement of the position of the substrate table, leading to overlay errors in exposures. U.S. Pat No. 6,232,051 and U.S. Pat. No. 5,331,371 disclose the use of masking blades in step and repeat type lithographic apparatus to prevent unwanted radiation from falling on the substrate table during the exposure of edge dies but do not suggest how the problem may be overcome in a scanned exposure.
Also, in some cases, alignment marks may be located in the mouse bites and these are protected from subsequent exposures. As shown in FIG. 2 of the accompanying drawings, to expose dummy structures in the part of the mouse bite not occupied by the alignment marker P2 whilst not exposing the alignment marker, requires the performance of two exposures 11, 12 to expose an L-shaped area on the substrate bordering the whole dies C but not covering the alignment marker P2. The necessity to perform two exposures is an undesirable loss of throughput. U.S. Pat No. 5,760,881 discloses shields at substrate level that can be moved over the substrate to protect alignment marks.